It would be strange to claim to have invented something that already existed, but whose status exists only because people were unaware of it prior. People did not say that they invented gold when it was discovered for instance.
In latin, discoverer is translated as inventor. They means the same thing. In French, somebody discovering a treasure can be called "l'inventeur du trésor".
If you really want to examine whether mathematics is discovered or invented, you very quickly become immersed in philosophy. It's an interesting conversation to have once or twice, but I'm a tad too drunk for that tonight. ;)